Liquid crystal devices have a wide range of applications. Their most well known use is information display devices called Liquid Crystal Displays (LCDs). LCDs display information in the form of simple alphanumeric characters through to complex streaming video. Common examples of these LCDs are found in digital watches, digital gas pumps, personal digital assistants (PDAs), laptop computer screens, and televisions. There are many other types of applications in which liquid crystal devices are used, such as: near-to-the-eye Head Mount Displays systems, projection systems, imaging systems, mastering and data storage systems, optical systems as shutters or light guides, variable retarders, variable attenuators, polarization rotators, spatial light modulators, beam steering devices, and filters.
In all of these applications that use liquid crystal devices, radiation is a common factor for user interface functionality. Some form of electromagnetic radiation (usually light) is being passed, blocked, or manipulated by a liquid crystal device. In certain applications, the electromagnetic radiation is far more intense and at wavelengths that have not typically been used in past applications. This higher intensity radiation, over a broader wavelength range or at a single wavelength, degrades materials comprising the liquid crystal device; most notably the alignment layer is significantly affected.
FIG. 1 shows one exemplary prior art holographic data writing system 10 that uses a liquid crystal device 15, a laser source 11, a beam splitting cube 13, a mirror 14, and a recording/storage medium 16. Laser source 11 outputs a high-energy beam of blue or ultra-violet (UV) light 12 that commonly has a narrow band wavelength of 405 nm, but may range down in wavelength to 250 nm. In the example of FIG. 1, beam splitting cube 13 splits light 12 into a first and a second optical path. Liquid crystal device 15 modifies the light of the first optical path, and mirror 14 reflects the light of the second optical path to combine with the modified light of the first optical path at recording/storage medium 16. Other optical elements may be included within the first and second optical paths and are not shown for clarity of illustration. For example, one or more of a collimator, a wave plate, a polarizer, a retarder, a prism, and a photodiode may be included within holographic data writing system 10. Light of the first optical path causes instability and degradation of liquid crystal device 15, since these prior art liquid crystal devices typically use an organic thin film polymer for the liquid crystal alignment layer.
FIG. 2 shows a top view of one exemplary prior art liquid crystal device 17. Liquid crystal device 17 has a liquid crystal device cavity 24 that is defined by a perimeter gasket 18 (e.g., an adhesive seal) and has interlaced pixel columns 19 defining clear aperture or active region 25. Liquid crystal device 17 also has contact ledges 20 and 21 for electrical connectivity to driving circuitry (not shown) via a flex cable 22 for example. Flex cable 22 has anisotropic conductive adhesive traces 23 that align with ITO traces of contact ledges 20, 21 of liquid crystal device 17. Flex connection to these ITO traces on liquid crystal device 17 is often made using both heat and pressure. An opposite end (not shown) of each flex cable 22 is often attached to a printed circuit board (PCB) containing the driving circuitry.
Different applications of this liquid crystal device involve different methods of use and different types of radiation for manipulation by the device. In certain applications the entire clear aperture of the device may be flooded with a high intensity broad wavelength range of electromagnetic radiation. In other applications, only a very small region of the device is exposed to high energy radiation, such as a 300 micron diameter single wavelength blue laser beam.
Liquid crystals, when built into a liquid crystal device, are very sensitive to the surfaces on which they align. These surfaces determine the direction, anchoring strength, and the pre-tilt (the amount of tip in the long chain molecules that are contacting the surface) of the liquid crystal molecules inside the device. In most cases, these surfaces on which they align are formed of an organic, polymer film called polyimide.
In a liquid crystal device, an alignment layer aligns the long direction of liquid crystal molecules to the direction of the rub treatment. The molecules at the surface also require a small amount of tilt (called pre-tilt) such that when the liquid crystals are manipulated with electric or magnetic stimuli, the average motion of all of the liquid crystal molecules is in the same direction. Pre-tilt is a critical aspect in obtaining sufficient liquid crystal alignment. Most liquid crystal displays and devices today utilize a thin polymer film, called polyimide, as the alignment thin film. The film is coated on top of the transparent conductive oxide (TCO) (typically ITO). The utilization of this material in liquid crystal device construction is reasonable from both cost and process perspective. Polyimide allows for consistent and repeatable liquid crystal alignment in association with the industry standard rubbing process. However, polyimide is absorptive of UV and blue wavelengths of light, and readily degrades in these wavelengths because it is an organic material. The amount of absorption of a polyimide film is related to variables such as polyimide type, film thickness, and crosslink density of the film.
Alignment of liquid crystals on rubbed oxides has been studied in the past, M. Nakamura, J. Appl. Phys. 52(7), July 1981, 4561-4567. Generally, the TCO that is used as an electrode layer in liquid crystal devices is a dense crystalline structure and is resistant to scratching and abrasion damage. The typical alignment layer processing, called rubbing, does not allow for liquid crystal alignment on this type of surface. Abrasives and oxide powders may be used to obtain alignment on more dense oxide films; however, the quality of the alignment obtained has proven unacceptable, thus requiring the use of the polyimide material.
Degradation of the polyimide material causes changes in the pre-tilt and anchoring strength of the liquid crystal molecules at the surfaces, which in turn changes the unbiased retardance of the liquid crystal device, and also causes changes in the electro-optical response of the liquid crystal when a bias is applied. Speed of the polyimide film degradation is dependent on factors such as wavelength, energy density, exposure length, liquid crystal type, and mode. In UV and blue wavelengths with high energy densities, changes in the liquid crystal electro-optic properties may be realized in a matter of hours, and complete failure may be realized in hundreds of hours. This failure rate is unacceptable in many systems, and users require a more robust solution. The liquid crystal material itself is subject to degradation. It is likely that certain formulations of liquid crystal will offer improved robustness to degradation, however; this robustness will be dependent on source wavelength and energy density. Liquid crystal absorption may be measured using a spectrometer in the region of interest of wavelength. By using this metrology technique one may draw absorption comparisons between different liquid crystal types or formulations. The liquid crystal material is also suspect to contamination, some of which may be generated from the degradation of the polymer alignment layer. U.S. Pat. No. 7,184,109, incorporated herein by reference, shows novel approaches to dealing with contaminated liquid crystal inside a cell.
It is known that polyimide decomposes when exposed to UV wavelengths of light and it is also known that it will degrade even within the visible spectrum at wavelengths between 400-500 nm. It is generally observed that liquid crystal devices are stable within the visible light spectrum between 500 nm and 700 nm. However, there is limited data of device performance and stability in high energy exposure within 500 nm to 700 nm, and particularly beyond 700 nm into the infrared (IR) spectrums. Liquid crystal material itself is organic in composition and may also decompose under intense electromagnetic radiation. When used with electromagnetic radiation of blue and UV wavelengths, the polyimide layer absorbs energy that decomposes the polyimide film. Because of the sensitivity of liquid crystal at the alignment surfaces, the liquid crystal response to applied voltage is affected by degradation changes occurring in the polyimide layer when it is continually exposed to high-energy radiation. Degradation of liquid crystal devices include: 1) a change in liquid crystal pre-tilt at the surface(s); 2) loss of liquid crystal alignment at the surface(s) due to a reduction in liquid crystal anchoring strength, and increased ionic contamination caused from pre-existing mobile ions in the liquid crystal and/or thin film layer(s); and 3) ions generated from the degradation of the alignment layer, or a combination of both ion contamination types. Any of these changes, alone or in combination, may affect the unbiased retardance, as well as the electro-optic characteristics, of the liquid crystal device within the area of electromagnetic radiation bombardment, which result in unstable and undesirable changes experienced by a user of the liquid crystal device.
Liquid crystal device stability and resistance to degradation are requirements for many applications. Subtle changes and instability in liquid crystal devices are unacceptable to many users of these devices. Given the use of higher energy density electromagnetic radiation at more destructive wavelengths, as required by emerging applications, an improved and robust liquid crystal device solution is needed.
The current technique for improving the liquid crystal device stability and resistance to degradation is to replace the organic polymer alignment layer with an inorganic material to which the liquid crystals properly align. Examples of such inorganic materials are silicon oxide and silicon dioxide. Silicon dioxide is typically deposited using an ion beam vacuum deposition process, where the substrate is typically positioned at some incident angle from normal. It is the geometric columnar structure and directionality of this deposited film that allows for adequate alignment of the liquid crystal molecules to the film. Obliquely deposited silicon oxide has been used in the construction of liquid crystal devices for many years, and was the main method for obtaining alignment before the invention of polyimide materials, and associated thin film processing. A description of silicon dioxide processing may be found in a paper by John L. Janning, “Thin Film Surface Orientation for Liquid Crystals”, Journal of Applied Physics, Vol 21, No. 4, 1972.
It has been proven that an inorganic silicon dioxide film, when used as an alignment layer, improves device stability and delays the degradation when exposed to UV radiation as described by Wen et al., Journal of the SID Sep. 13, 2005, 805-811. However, there are several shortcomings with this alignment layer solution. The deposition process is time consuming and requires expensive equipment. Many coating recipes also require two or more separate coating runs at potentially different thicknesses. Variations in the film(s) become increasingly difficult to control as substrate size increases, and substrates are prone to contamination. Thus, the realized improvements in the liquid crystal device may not outweigh the costs in achieving them. The exposure conditions for testing device stability and longevity are much harsher than what is seen in actual applications. The problem with real time and true source testing is that it may take considerable time to obtain results; acquiring sources represented in actual applications may be expensive. The results in many life tests have been accelerated, making it difficult to extrapolate the degradation result to a real time application. The instabilities reported by Wen et al. (2005) are measured and tested against polyimide samples, and all samples experience short-term exposure (less than 200 hours). Contemporary liquid crystal device users require 1,000 to 10,000 hours of device stability, so improvements to device longevity and stability of a few hundred hours likely will not apply to an application requiring thousands of hours with little or no change.